A transmission unit of a communication device, such as a cellular phone or a wireless LAN, is required to operate with low power consumption. Such a transmission unit of a communication device is required to operate with low power consumption regardless of the magnitude of output power, and is also required to ensure a high accuracy of transmission signals. In particular, a power amplifier disposed at a final stage of a transmission unit of a communication device occupies 50% or more of the power consumption of the entire transmitter. For this reason, the power amplifier disposed at the final stage of the transmission unit of the communication device needs to have a high power efficiency.
In recent years, a switching amplifier has been attracting attention as a power amplifier that is expected to achieve a high power efficiency. The switching amplifier receives pulse-shape signals. The switching amplifier amplifies the power of received pulse-shape signals while maintaining the pulse shape. The pulse-shape signals amplified by the switching amplifier are transmitted from an antenna after frequency components other than desired frequency components are sufficiently suppressed by a filter element.
A current mode class-D amplifier (hereinafter referred to as “CMCD”), which is a typical switching amplifier, will now be described. FIG. 17 is a circuit block diagram showing a configuration example of a CMCD 6. The CMCD 6 includes variable current sources 61 and 62, switching elements 63 and 64, a load 65, and a filter circuit 66. The variable current sources 61 and 62 are connected in parallel with a power supply VDD. The switching element 63 is connected between the variable current source 61 and a ground GND. The switching element 64 is connected between the variable current source 62 and the ground GND. The load 65 and the filter circuit 66 are connected in parallel between an output terminal of the switching element 63 and an output terminal of the switching element 64.
Pulse signals are input to control terminals of the switching elements 63 and 64. The pulse signal input to the control terminal of the switching element 64 is a signal complementary to the pulse signal input to the control terminal of the switching element 63. Thus, when one of the switching elements 63 and 64 is turned on, the other of the switching elements is controlled to be turned off. The current source connected to the switching element in the OFF state supplies a current to the load 65 and the filter circuit 66. A current from the current source connected to the switching element flows into the switching element in the ON state. Further, a current from the current source connected to the switching element in the OFF state flows into the switching element in the ON state through the load 65 and the filter circuit 66.
A typical class-AB power amplifier and the like require a bias current. On the other hand, the CMCD requires no bias current. Accordingly, a power loss in the CMCD is equal to the sum of a switching loss generated during charging/discharging to/from a parasitic capacitance and a heat loss generated in a parasitic resistance. Accordingly, when the parasitic capacitance and the parasitic resistance are ideally zero, the power loss of the CMCD is “0”.
Next, a configuration example of a transmitter 700 incorporating the CMCD 6 will be described. FIG. 18 is a circuit block diagram showing a configuration example of the transmitter 700 incorporating the CMCD 6. The transmitter 700 includes an RF signal generator 71, a driver amplifier 72, and the CMCD 6.
The RF signal generator 71 includes a digital baseband (hereinafter referred to as “DBB”) 711, sigma-delta modulators 712 and 713, a digital up-converter 714, and an inverter 715. In the case of W-CDMA, for example, the DBB 711 generates a radio signal which is a multi-bit signal of 10 bits or more. On the other hand, a pulse signal representing information by two states (1 bit) of high and low can be input to the CMCD 6. Accordingly, it is necessary to convert the multi-bit signal output from the DBB 711 into a 1-bit signal that is subjected to over-sampling in advance. The sigma-delta modulators 712 and 713 are used as means for converting a multi-bit signal into a 1-bit signal by over-sampling.
Each signal output from the sigma-delta modulators 712 and 713 is output as a pulse signal through the digital up-converter 714. The pulse signal output from the digital up-converter 714 is divided into two signals. One of the divided pulse signals is inverted by the inverter 715. A non-inverted pulse signal is input to the switching element 63 of the CMCD 6 through the driver amplifier 72. The inverted pulse signal is input to the switching element 64 of the CMCD 6 through the driver amplifier 72.
The sigma-delta modulators 712 and 713 described above can favorably maintain noise characteristics in the vicinity of a desired frequency band. Accordingly, in this configuration example, a multi-bit radio signal can be converted into a pulse signal and the pulse signal can be input to the CMCD 6, while maintaining satisfactory noise characteristics.